Plaster patching on wood lath ceilings can be carried out by different methods. Assuming the patch is approximately one meter square, the repair can be made good by nailing a plasterboard, cut to size and shape of the patch, to the ceiling joists. The surface of the plasterboard is skimmed to bring it in line with the original plastered surface, making good the piecing as before described. Where sufficient thickness is available, the ends of the board can be scrimmed to avoid the possibility of a cracked ‘joint’.
If the thickness available will not permit the use of a plasterboard nailed to the existing wood laths, then these are removed carefully on the centre line of the outside joints exposed in the patch area. This will mean that inevitably some making good to the full thickness of the original plastering will have to be carried out on the perimeter of the patch before skimming can take place.
In situations where the use of plasterboard is not possible or desirable, the patching can be done in the following manner. The surrounding edge of the patch can be tested for soundness and any loose unkeyed or defective plaster removed by hand, the plasterer standing underneath the sound part of the ceiling.
Any broken or missing laths should be replaced at this stage. Dust should be removed from the laths by sweeping with a broom away from the operative. The piecing only should be damped. The re-plastering can be done with a line/sand/gypsum plaster or vermiculite/gypsum plaster mixes. The rendering or pricking up coat should be applied to the perimeter edge first and then diagonally across the laths away from the operative.
It is important that sufficient material goes through the lath joints to form keys above the laths and, at the same time, retaining sufficient under the laths to form a continuous bond in one application. The area should be floated out to full thickness in one operation but often the pricking up coat has to be keyed and allowed to set before the floating coat is applied. The floating coat is straightened, scraped back for skimming clearance and rubbed up in the same way as for wall patching. Skimming coat on lime/sand/gypsum plaster undercoats should be lime putty and plaster. Skimming coats on vermiculite/gypsum plaster should be lightweight aggregate/gypsum finish plaster.
Patching on plasterboard ceilings can be carried out by removing the broken plasterboards to the centre line of the outermost exposed joists in the patch area. A new plasterboard or boards are cut to fit and nailed in the space exposed by removed boards. The replacement plasterboards are then skimmed with Board Finish plaster and the piecing made good as described previously.
An easier method is to cut a piece replacement plasterboard about 100 mm wider and longer than the hole in the plasterboard ceiling. (The width of the replacement board must be narrow enough to pass through the hole in the ceiling.) To fix the plasterboard patch it is first necessary to clean the upper patch perimeter clear of the dust. Two small holes are drilled into the replacement plasterboard and a length of string is threaded through near the middle. The upper side of the patch perimeter edge is coated with a neat plaster mix and the replacement board is bedded into the soft mix to cover the gap.
This is achieved by threading the replacement board up through the hole in the ceiling, turning it to cover the whole of the patch and then pulling the replacement board down tight to the bedding coat with the string. (An alternative method is to use two roundhead nails and to pull the board down by gripping the nail shanks.) When the bedding has set, a further piece of plasterboard can be cut to fill the recess, and this is bedded tightly against the replacement board leaving a normal skimming thickness to make good afterwards.
Another variation on this method is to bed the recess filling plasterboard to the larger replacement before bedding the combined boards in position to cover the gap in the ceiling. The string used for pulling the replacement board down on to the bedding mix is removed before skimming and making good the piecing.